As the world's demand for electric energy increases, the existing power grid is being taxed beyond its ability to serve this demand continuously. In certain parts of the United States, inability to meet peak demand has led to inadvertent brownouts and blackouts due to system overload as well as to deliberate “rolling blackouts” of non-essential customers to shunt the excess demand. For the most part, peak demand occurs during the daytime hours (and during certain seasons, such as summer) when business and industry employ large quantities of power for running equipment, heating, air conditioning, lighting, etc. During the nighttime hours, demand for electricity is often reduced significantly, and the existing power grid in most areas can usually handle this load without problem.
To address the possible insufficiency of power supply at peak demand, users are asked to conserve where possible. Also, power companies often employ rapidly deployable gas turbines to supplement production to meet peak demand. However, these units burn expensive fuels, such as natural gas, and have high generation costs when compared with coal-fired systems and other large-scale generators. Accordingly, supplemental sources have economic drawbacks and, in any case, can provide only a partial solution in a growing economy. The most obvious solution involves construction of new power plants, which is expensive and has environmental side effects. In addition, because most power plants operate most efficiently when generating a relatively continuous output, the difference between peak and off-peak demand often leads to wasteful practices during off-peak periods, such as over-lighting of outdoor areas, as power is sold at a lower rate off peak. Thus, it is desirable to address the fluctuation in power demand in a manner that does not require construction of new plants and can be implemented either at a power-generating facility to provide excess capacity during peak, or on a smaller scale on-site at the facility of an electric customer (allowing that customer to provide additional power to itself during peak demand, when the grid is heavily taxed).
Additionally, it is desirable for solutions that address fluctuations in power demand to also address environmental concerns and support the use of renewable energy sources. As demand for renewable energy increases, the intermittent nature of some renewable energy sources (e.g., wind and solar) places an increasing burden on the electric grid. The use of energy storage is a key factor in addressing the intermittent nature of the electricity produced by some renewable sources, and more generally in shifting the energy produced to the time of peak demand.
Storing energy in the form of compressed air has a long history. Most methods for converting potential energy in the form of compressed air to electrical energy utilize turbines to expand the gas, which is an inherently adiabatic process. As gas expands, it cools off if there is no input of heat (adiabatic gas expansion), as is the case with gas expansion in a turbine. The advantage of adiabatic gas expansion is that it can occur quickly, thus resulting in the release of a substantial quantity of energy in a short time.
However, if the gas expansion occurs slowly relative to the time which it takes for heat to flow into the gas, then the gas remains at a relatively constant temperature as it expands (isothermal gas expansion). Gas stored at ambient temperature that is expanded isothermally provides approximately three times the energy of ambient-temperature gas expanded adiabatically. Therefore, there is a significant energy advantage to expanding gas isothermally.
In the case of certain compressed-gas energy-storage systems according to prior implementations, gas is expanded from a high-pressure, high-capacity source, such as a large underground cavern, and directed through a multi-stage gas turbine. Because significant, rapid expansion occurs at each stage of the operation, the gas cools at each stage. To increase efficiency, the gas is mixed with fuel and the mix is ignited, pre-heating it to a higher temperature and thereby increasing power and final gas temperature. However, the need to burn fossil fuel (or apply another energy source, such as electric heating) to compensate for adiabatic expansion substantially defeats the purpose of an emission-free process for storing and recovering energy.
A more efficient and novel design for storing energy in the form of compressed gas utilizing isothermal gas expansion and compression is shown and described in U.S. patent application Ser. No. 12/421,057 (the '057 application), the disclosure of which is hereby incorporated herein by reference in its entirety. The '057 application discloses a system for expanding gas isothermally in staged hydraulic/pneumatic cylinders and intensifiers over a large pressure range in order to generate electrical energy when required. The power output of the system is governed by how fast the gas can expand isothermally. Therefore, the ability to expand/compress the gas isothermally at a faster rate will result in a greater power output of the system.
While it is technically possible to attach a heat-exchange subsystem directly to a hydraulic/pneumatic cylinder (an external jacket, for example), such an approach is not particularly effective given the thick walls of the cylinder. An internalized heat exchange subsystem could conceivably be mounted directly within the cylinder's pneumatic (gas-filled) side; however, size limitations would reduce such a heat exchanger's effectiveness and the task of sealing a cylinder with an added subsystem installed therein would be significant, making the use of a conventional, commercially available component difficult or impossible.
A novel compressed-gas-based energy storage system incorporating an external heat transfer circuit is disclosed in U.S. patent application Ser. No. 12/481,235 (the '235 application), the disclosure of which is hereby incorporated herein by reference in its entirety. The '235 application discloses a hydraulic/pneumatic converter component in a staged energy storage system that can store high-pressure gas at, for example, over 200 atmospheres (3000 psi) for use by the system. A pressure vessel or cylinder defining a gas chamber (pneumatic side) and a fluid chamber (hydraulic side) has a piston or other mechanism that separates the gas chamber and fluid chamber, preventing gas or fluid migration from one chamber to the other while allowing the transfer of force/pressure between the chambers. Both the gas chamber and the fluid chamber have primary ports that interface with the respective pneumatic and hydraulic components of the overall energy storage and recovery system. The gas chamber/pneumatic side of the cylinder has additional ports. The additional gas exit port is in fluid communication with an inlet to a circulation device (for example, a pneumatic pump or fan impeller), the exit of which is in fluid communication with the gas inlet of a heat exchanger. The gas exit port of the heat exchanger is in fluid connection with the additional gas chamber inlet port. The heat exchanger has corresponding fluid ports that support a flow of ambient-temperature fluid through the heat exchanger in a direction counter to the flow of gas in the heat exchanger. Thus, due to the heat exchange with the flowing fluid, the gas exiting the heat exchanger is returned to the gas chamber at ambient or near ambient temperature. (The term “ambient” is used to represent the temperature of the surrounding environment, or another desired temperature at which efficient performance of the system may be achieved.) The circulation of gas in the gas chamber through the heat exchange subsystem thereby maintains the gas in the gas chamber at ambient or near-ambient temperature. The entire gas circuit in the heat exchanger is sealed and capable of handling high pressures (e.g., 200 atmospheres) encountered within the pneumatic side of the cylinder. The fluid side of the heat exchanger communicates with an appropriate reservoir of ambient fluid.
However, the prior art does not disclose systems and methods for increasing efficiency and power density in isothermal compressed-gas-based energy storage systems having heat exchangers by heating or cooling the heat-transfer fluid.